In motor vehicle exhaust gas treatment systems particle filters serve to filter particles, such as soot particles, from the exhaust gas. They are used for example in diesel internal combustion engines or spark ignition engines of motor vehicles. It is desirable to monitor the function of the particle filter so that in the event of a filter defect appropriate countermeasures can be taken. Up to now the functioning of such particle filters has been monitored in particular by differential pressure sensors, which measure a pressure difference upstream and downstream of the particle filter. If such differential pressure measurements indicate values that are critical for the functioning of the filter, a regeneration of the particle filter is usually initiated. A diagnosis based on differential pressure cannot, however, provide clear information about the efficiency of the particle filter. In particular it is not possible with such differential pressure measurements to reliably detect some defects of the filter, such as small holes in the filter. The California Air Resources Board (CARB), for example, would therefore like to see the development of a particle sensor to measure and monitor the functioning of the particle filter. Such a sensor is, however, complex and therefore costly.